Generally, an electronic device, such as a personal computer, operates from a direct-current power supply. Therefore, for example, a power supply adapter is used to convert commercial alternating current into direct current and supply the direct current to an electronic device.
To configure an electronic device to consume less energy, the supply of electricity to the electronic device from a power supply adapter is now being reexamined.
In recent years, electric power generated by solar power generation, wind power generation, or like is stored in a second battery temporarily. The direct current stored in the secondary battery is converted into alternating current by, for example, an inverter. The alternating current is then supplied to a distribution system. The conversion efficiency of the inverter has been improved but is not sufficient, leading to a conversion loss. In addition, the electric power converted into alternating current is converted into direct current by a power supply adapter to supply electricity to an electronic device. In this case, too, a conversion loss occurs. Therefore, it is desirable that the electric power generated from natural energy should be supplied in direct current to the electronic device.
Furthermore, an electronic device is used in, for example, a car, it is desirable that the direct current generated in the car should be used without the conversion of the direct current into alternating current. However, a power supply adapter serving as a power supply circuit used in an existing electronic device has only the function of converting alternating current into direct current. For this reason, it is difficult for the power supply adapter to handle a direct-current power supply. Accordingly, a power supply circuit and power supply control method which can output required direct current even when either alternating current or direct current is supplied have been desired.